Quartz crystal and method of treating same



Aprll 18, 1950 N. A. WOOSTER ETAL QUARTZ CRYSTAL AND METHOD OF TREATINGSAME 4 Sheets-Sheet 1 Filed Feb. 12, 194'! INVENTORS. NORA A. WOOSTERWILLIAM A. WOOSTER BY EDWARD A FIELDINVG ATTOR Y April 18, 1950 N. A.WOOSTER E IAL QUARTZ CRYSTAL AND METHOD OF TREATING SAME Filed Feb. 12.-1947 4 Sheets-Sheet 2 z and Z INVENTORS, NORA A. WOOSTER WILLIAM A.WOOSTER ATTO NEY April 18, 1950 N. A. WOOSTER ETAL ,5

QUARTZCRYSTAL AND METHOD OF TREATING SAME Filed Feb. 12, 1947 4Sheets-Sheet 3 INVENTORS. NORA A. WOOSTER WlLLlAM A. WOOSTER Wow?FIELDING %T0R Ny A ril 18, 1950 N. A. WOOSTER ET AL QUARTZ CRYSTAL ANDMETHOD OF TREATING SAME I 4 Sheets-Sheet 4 Filed Feb. 12, 1947 FIG. I60

INVENTORS. NORA A. WOOSTER WiLLIAM A.WOOSTER ATTOR Y the frequency of anoscillator.

iaten ted Apr. 1 8 195 UNITED Ares fe PiATENT OFFICE QUARTZ CRYSTALANDMETHOD. OF

1 TREATING SAME N ra na .Wocsic a .wi iamA red W90 Cambridge, and EdwardArmitage Fielding,

, Birch;England,-assignors toThe-General-Elec- .tric Company; Limited,London, England Application February 12,1941, Serial No. 728,030

In Great Britain September 14; 1943 Sectionl, Public Law 690, August8,1946

Patent. expires September; 14,1963

13 Jlaims. (Cl. 171-327) 1 This invention relates to methods of'treatingquartz crystals, which. manifest twinning of the type known aselectrical twinning, and tothe .lquartz crystalsproduced bysuch methods.".The

word crystal, as used herein with:reierence to i the treatment inaccordance with the-invention, refers to untreated Qrtreated portionscontaining only part of .the structure of an ideal orlcompletelydeveloped crystal. HTheseportions of a crystal .may-takethe forms 0tplates,.bars, or

.the like such as areused as piezoelectric elements,

for example, elements oi the type used to control Although -acrystalwhich exhibits twinning actually includes two or more crystalline unitsseparated byboundaries and. having the related crystallographicstructures peculiar to twinning, the twinned structure will be referredto as a crystal.

- Hitherto the usual treatment of raw quartz crystals,- such as natural.quartz,-included mere1y -optical. determination of the crystalstructure -and-cutting and trimming to obtain flawless por---tions-having the-desired size and orientation-with respect to thecrystallographic axes, It is known,

however, that quartz; otherwise of good quality, 'may be unsatisfactoryfor the manufacture of A piezoelectric elements because of twinning.

Twinningmay be oftwo k-inds, optical and-electrical, and a twinnedcrystal mayshow-either form oftwinning separately, or both forms oftwinning .mayco-exist simultaneously-9 The presence of twinning-leads towaste and-expense in.the manufacture of .quartzcrystals, since, out

of a given batch of quartz, an appreciablapro- ,.portion may. have to.be discarded as unsuitable.

-...Accordingly, an. object ofathis invention is to ....provide a newand improved method of...treating quartz .crystals .which substantiallyavoids..0ne

.or more of the, limitations ofthedescribedprior .art treatment.

- It is a;f urther object of the i-inv ention tq.re-

e tq ed eer efix e Q tri a ii ning in quartz crystals so as. to reducerthe amounts of unusable. quartz hitherto wastedtdue to ,the defectsattributable to. electrical twinning.

It is also an object oithisinyention to provide ,new and o dqua z c t lha nea m ed P t 9 l ct i tw nni In accordance with the invention,amethod of treating a quartz crystal comprises heating the crystal andthereafter cooling the crystal l while thecrystal is strained to modifythe pat- 1 tern of the electrical-twinning; I-naccOrda-nce .with onefeature of the.- invention, .a .quartz crysatal whichcmanifestselectrical twinning istreated toproduce a modification of the pattern oftwinningz which serves to reduce. .theproportion of -,the volume of. the,crystal which is electrically twinned.

' In accordancewith a-preferred embodiment of 1 the invention,. themethod comprises, heating the crystal to a temperature appreciably aboveits a1pha-beta inversion temperature to, convert it to..the .beta form,.andthereafter, cooling the .crystalthrough its inversion temperaturewhile vlthe crystal is strained. Itwill. be understood :by those skilledin the artthat, if. a quartzpiezo- .electriccrystal isheated to atemperatureap- .precia'bly above..the.equilibrium temperature of-.inversion. betweenthe. alpha ((1) and beta. (,8) phasesaandthis-temperature is. maintained for a sufficienttime to permitequilibriumconditions to be established or. approached, all parts of.the

crystalmay beconverted to the beta form. .The inversion temperature isabout 573 C.

For a better understanding of the present invention together with-otherand further objects thereof, reference is had to-thefollowingdescription taken in connection with the accom--.panying-drawings, and its scope will be pointed out in the appendedclaims.

i In the drawings, Figs. 1 and2 are perspective Views ofrespectivelyleft-handed and righthanded crystals in a'fully developedcondition;

Figs. 3 and 4 are: horizontal sectional views-taken "throughcentralportions ofthe crystals of;Figs.

l and Z respectively; Figs 5a, 5b, 5c, d 5111 diagrams illustrating theconfiguration of the co-ordinate axes used conventionally in describingcrystal sections cut from crystals ofthe type tional viewstaken at theplanewAA of Fig. land illustrating various types of twinningwhich mayoccur; Figs. 7a, 7b, '70, 8a, 8b, and 8c are Vi ws of sectionscutfrom'the crystal of Fig. 1, assuming twinning therein of the typesillustrated in Figs. 6d,,6b, and 69 respectively, and illustrating thecorresponding"twinning effects occurring in such sectionspFig. 9comprises seven'views, designated a- -g, representing diagrammaticallythe structure of treated crystals cut with faces haying seven differentorientations with respect to their crystallographic axes; Fig. 10 isaperspectiveview' of; apparatus used in carrying-out one embodimentoi thetreatment in accordance with the invention; and Figs.- 11 12, 13, 14,15,

i and 16, each of which'includes three views designated a, b, and c,aresimplified representations of. portions of crystals manifestingtwinning and include; representations of such. portions; before one ofthe electric axes of the crystal.

f indicated in each case. I -symmetry of alpha-quartz it is immaterialwhich of the three symmetrical electric axes is taken as theX-direction. The fact that the sections are not regular hexagons hasbeen ignored as it is immaterial to the discussion. It will be seen thatthe Y-axis coincides with one of the so-called me- -rcnanical axes ofthe crystal, which has electrical and after treatments of the typedescribed herein.

Treatment of the kind here proposed is not invariably successful, forcrystals of certain cuts are more difiicult to treat than others, andthe precise circumstances of th treatment must to some extent bedetermined by trial on a specimen of the quartz which it is desired totreat. But, since some of the more important commercial cuts, includingthe AT, BT, and CT cuts. are among those which can comparatively readilybe treated, economy in large scale manufacture can be achieved byintelligent use of the invention. More than one method of producing therequired and mechanical axes as well as the optical crystallographicaxis.

The method used for specifying the position, relative to the X-, Y-, andZ-axes, of a small rectangular quartz plate cut from the left-handedcrystal of Fig. 1 is shown in Figs. a, 5b, 5c, and 5d. Suppose that theplate is actually out in the position indicated in Fig. 501. Itsposition is constrain may be employed, for'it may be produced either byimposing a temperature gradient on the quartz, or by the directapplication theretov of a mechanical stress such as a torsional couple,"or.

by a combination of temperature gradient and mechanical stress. a

In order that the circumstances may be set out adequately, it will beconvenient to give in detail a suitable set of co-ordinates forspecifying the configuration of a sample cut from a crystal,

in relation to the crystal from which it is cut,

It is well known that from the optical point of .view alpha-quartz ma beof two for-ms, one form :31? showing a left-handed rotation, whenlooking toward the light source, of the plane of polarized I .lightpassed through the crystal in the direction of the optical axis, and theother form showing right-handed rotation. These forms will herein xbereferred to as left-handed and right-handed forms of quartz,respectively. The optical axis will be taken as the Z-axis of a set oforthogonal axes, and the X-axis of the set will be chosen at rightangles to the Z-axis and in the direction of The positive direction ofthe X-axis is so chosen that, if

I a compressional stress be applied to the ends of that axis, a positivecharge is developed at the y a positive end of the axis and a negativecharge at the negative end of the axis. The Y-axis is taken i at rightangles to the Z- and X-axes, the positive direction of the Y-axis beingchosen in a manner to be described in connection with the drawings.

The conventions regarding the positive direction of the Y-axis will bereadily understood with 7; reference to Figs. 1 and 2 of theaccompanying drawings, in which Fig. 1 is a view of an ideal left-handedcrystal, while Fig. 2 is a view of an ideal right-handed crystal, thearrangement of Conversely the positive direction of tal of Fig. 2 ischosen to form what ma be called a left-handed set of axes. Figs. 3 and4 show sections in the plane Z=O through the crystals of Figs. 1 and 2respectively, the signs of the charges produced by compression along theX-axis being Owing to the trigonal veniently specified by giving thesuccessive rotations which an imaginary rectangle originally lying inthe XY plane with its longest dimension or length parallel to theX-axis, as shown in Fig. 5a, would hav to undergo in order that it mightbe brought into coincidence with the central plane of the plate. Theserotations are as follows: first, a rotation about the Z-axis through anangle- 1) so that the axes OX, OY of Fig. 5a

are brought into the position shown in Fig. 5b, where the new positionsof OX and OY are indicated by OX, OY'for symmetry, the position of OZ,although unchanged, is labelled OZ; then a rotation through an angle 0about OX, so that OY and OZ are carried into the positions CY" and OZ"shown in Fig. 5cagain, for symmetry, the position of OX, althoughunchanged, is labelled OX in Fig. 50; and finally a rotation about theaxis OZ" through an angle 1.0, so that OX" and OY are carried to thepositions OX" and CY" of Fig. 5d.

It is noted that each of the rotations should be made in the indicatedsense and, in the case of 0 and it, each may be of any magnitude from 0to 180. In the case of however, because of the trigonal symmetry, theX-axis is always chosen to coincide with that one of the threeelectrical axes which is such that the magnitude of the required-rotation is not greater than In the case of a right-handed crystal, theprocedure follows a corresponding course of rotation of the imaginaryrectangle through an angle v about OZ, rotation through an angle 0 aboutthe OX axis so produced, and then rotation through an angle \l/ abouttheOZ" axis so produced, but in this case each rotation is made in theopposite sense, so that, just as in the case illustrated in Figs. 5a,5b, 5c, and 5d, a positive angle for example, corresponds to a rotationabout OZ in a sense which carries OX toward the instant position of OY.

The nature of the defects which arise from the various possible forms oftwinning will now be explained with reference to Figs. 6a, 6b, 60, 7a,7b, '70, 8a, 8b, and 8c of the accompanying drawlngs.

Let it now be supposed that, in Fig. 1, the material of the crystalwithin a central core thereof is twinned. Figs. 6a, 6b, and 60 show bythe outline'2l the section of the surface of the crystal in the plane AAof Fig. 1, and also show by the outline 22 the section of this core ofthe crystal in the plane AA. In Figs. 6a, 6b, and 6c, the fact that thecrystal sections in the plane AA are not regular hexagons has again beenignored, and

the outlines 2| and 22 have been drawn as though the hexagons wereregular. Owing to the nature of the structure of quartz, thearrangements of the atom in the faces indicated by the reference lettersR and r in Fig. 1 difier from one another,

and the same reference letters are used in Figs. 6a, 6b, and Go toindicate the corresponding faces. The area of the twinned portion of thecrystal in the plane of the section 2| is indicated by shading in theregion 22 of each of these figures, and thesenses of the opticalactivity of the corre- .curved arrows. 1

sponding, parts ofthe crystal v are indicated by Fig. 6a relate to thecase where there electrical twinning, but where there is no opticaltwinning, as will be seen from the senses of the curved arrows. Theresult is that, if the crystal is compressed in the direction of theX-axis, positive and negative charges are developed .as shown at thepoints 23 and 24 of the outer portion and at the points 25 and '26 ofthe inner twinned portion. It will be observed-that, because of theelectrical twinning, the charges developed on the twinned portion are ofpolarity opposite to that of the charges developed on the outer portion.As has already been remarked,

any of the other electrical axes of symmetry might be taken as theX-axis, and if the crystal were compressed along one of those axes, forexample the axis Xi shown dotted in Fig. 6a, charges would develop onthat axis as shown at 21, 28, 29, and 30. It will be noted also that,where there is electrical twinning, the r faces on the twinned portionare parallel to the R faces on themain body of the crystal, andviceversa. Fig. 6b relates to the case where thereis optical twinning,as indicated by the opposed senses of the curved arrows, but noelectrical twinning. It will be seen that the R faces on the twinnedportion are now parallel to the R faces on the main body of thecrystahand two 1 facesare likewise parallel. In this case, compressionalong the X-axis shown produces positive charges at the points 3i and 32and negative charges at the points 33 and 34. Compression along eitherof the other possible directions for an X-axisproduces correspondingcharges at the appropriate points on the axis chosen. Fig. 6b, as wellas Figs. 6a and 60, indicates the signs of all the charges that may beproduced, though it willbe appreciated by those skilled in the art thatnot all i the charges are produced simultaneously by pressure along asingle electrical axis. As in the case illustrated in Fig. 6a, thecharges developedhon the inner twinned portion are polarized oppositelyto those developed on the outer portion.

Finally, Fig. 6c shows a case where there is compound optical twinning,i. .e,.-, both optical twinning, as indicated by the opposed senses ofthe curved arrows, and electrical twinning. In this case, which iscomparatively rare; an 1 face on the twinned portion is parallel to an Rface on the main body of the crystal, and vice versa.

Compression along an electricalaxisX, of which one only is shown,produces.positive charges at the points 35 and 36 and negative chargesat th points 31 and 38.

The types of defects resultmgfrom the various forms of twinning will beapparent from the foregoing description. They will be exemplified by theexamples given in Figs. 7a, 7b, and 7c and in Figs. 8a, 8b, and 8c.

,Figs. 7a, 7b, and '70 show sections, in a plane perpendicular to theZ-axis, of thin slices of crystal cut from the crystals to which Figs.6a, 6b, and 60, respectively, relate. The slices are to be understood tohave been cut from the original crystals so as to be bounded by the -twoplanes perpendicular to the plane of the paper indicated by the dottedlines 39:: and 40a, 39b and 49b, and 390 and 490 in Figs. 6a, 6b, and60, respectively. Such slices have their faces normal to one of theX-axes, e. g.,-the axis X1 in Fig. 6a;

, the positive. direction of this X-axis for the main positive directionof the X-axis for the twinned portions, again shown shaded, is indicatedby the arrows marked 42a, 42b, and 420, respectively. For these slicesthe angles 0, and 1/ ar 30, 90, and 0, respectively. r

It will be clear from the description already given that, if the sliceshown in Fig. 7a undergoes uniform compression in the X-direction of thelower part of the slice, the left-hand face of the lower part develops anegative charge while the right-hand face develops a positive charge, asillustrated. The opposite is the case with the upper twinned part, theright-hand face becoming negative and the left-hand face positive, asindicated in the drawing. Such a slice is unsuitable for use as apiezoelectric element.

Again, if the slice whose cross section is shown in Fig. 7b iscompressed in the direction of the X-axis of the lower part of theslice, a negative charge develops on the left-hand face of the lowerpart and a positive charge on the right-hand face, while a positivecharge develops on the lefthand face of the twinned upper part and anegative charge on the right-hand face, as illustrated. Such a slicesimilarly is unsuitable for use as a piezoelectric element. 7 Finally,if the slice whose cross section is shown in Fig. 7c is compressed inthe direction of .the X-axis, the signs of the charges developed on thefaces of the twinned and untwinned parts of the crystal will be thesame, as illustrated. Such a plate, however, in general shows defectsdue to its elastic inhomogeneity. It is only in cases in which a purelongitudinal vibration is excited; in a direction parallel to the lengthor the thickness of the plate, that vibration not being coupled to anyother mode of vibration, that effects due to the lack of homogeneity donot manifest themselves. Such deleterious effects arise ifshearvibrations are produced or are desired, asis ver often the case.

Figs. 8a, 8b, and 8c illustrate the corresponding phenomena for slicescut from crystalshaving twinned cores, similar to those representedrespectively by Figs. 6a, 6b, and 60 along planes parallel to an R faceof the main crystal. Figs. 8a, 8b, and 80 represent the iaces oftriangular slices cut from the crystal of Fig. 1, assuming it to have atwinned central core as shown in the slices on the horizontal sectionsof Figs. 6a, 6b,

and 6c are shown in the respective flgures. Of course, smaller plates ofdesired shapes may be cut from the triangular slices illustrated. It isthought that it will not be necessary to set-out the description infurther detail, but the signs of the charges developed when therespective slices are compressed in the direction of the X-axis oi themain portion of the crystal are again shown, and the discrepancy betweenthe signs of the charges developed on the edges of the two parts of thecrystal slices shown in Figs. 8a andBb are again apparent. Likewise, inthe case of the slice represented in Fig. 80, there is again dim cultybecause of discrepancy between the elastic: properties of the two partsof the crystal, despite the fact that there is no longer a discrepancyin; the signs of the charges.

In the foregoing description, it has been;

assumed for simplicity that-the boundaries between the main. portion of"the crystaland. the twinned portion are plane surfaces.- Inquartzcrystals as found in nature this 'isnot generally the case, theboundaries frequently being curved. mien a slice is out from a crystalwhich shows twinning, part of the slice will consist of material havingthe same structure as the main body or the crystal, and-part, whichmaybe inthe :form ora patch-orpatches bounded by the rein'ainder, willhave the twinned structure. By the treatment in accordance with theinvention, the proportion of the plate which is occupied byithe-electrically twinned material can trequently ibe reduce'd or mayeven be removed entirely, or the electrically twinned part may belocalized so ithat smaller plates free from twinning-can be outfro'm'the plate. As ra'r'as weare aware, the 'itreatment d'oes' notchange the sense of the optical activity-many part of the crystal;

4 When quartz is raised to a temperature appre- *ciably above the--alpha-beta inversion temperacure, it is converted from alpha-quartztothe forin-ofbeta-quar tz, there being a rearrangement "of tli'e atomscomposing the quartz such that while the alpha-quartzpossessedt-ri'gonal symin'etry; as previously mentioned, the beta-quartzpossesseshexagonal symmetry. The material has, in fact, undergone apolymorphic change. If the material is then allowed to cool downthroughthe inversion temperature, itreturns to the alpha form, thereverse polymorphic change fhav'ing then taken place. If, during theprocess of' raising its temperature above the inversion jtemperature andlowering it again slowly below ithat temperature, no, or but little,strain is imposed on a quartz crystal which shows electrical TOW-inning,the state of twinning of the crystal may aind'ergo some change but thechange isapparehtiyrandom-inits nature and noeffectivereductionin theamount of twinning will-result onthe average, 'nor'will any stablepattern of twinning "be )alttained. However, using the method 'oftreatment in accordance with the presentinven- -tion, comprising heatinga crystal which manirests electri'cal'twinning, with or without opticaltwinning present in thesame crystal, while the cryst'alis strained-thereresults a reversal of-the positive directionsof the electrical axes atleast' 'a portion of the crystal, whereby thepattern or the electrical"twinning is modified a'n'd' i'n most casestheproportion of thevolume'of the crystal *which is electrically twinned is reduced. Thus'quartz crystals are produced which manifest "1cCtriCaltwinning in whichthe patternof twinnin'g has been modified by' th'e treatment "d'e'efs'cribed, and preferably crystals are producedin 'which'the'proportion'of the crystal volume which f manifests electrical twinninghasbeen reduced ;by the trea'tment described.

'Inestim'a'tin'g what proportionof'the' volumeof the crystalislele'ctrically twinned-atany'stag'e -"of ;-':tieatment, it is to beunderstoodthat that profportion'is' to be measured bythe ratio o'fthevol- 'ume of that part cfthe crystal which is. of one "electrical"polarity to the volume which is of the 'other polarity, thelargervolumebeingchosenas ?"the denominator of the ratio. Thus,'forexampie, it may be "that, before trea'tmentfthestate "ofthe-majorpartof thevolume of the crystal is I such that; when compressed in the"direction" of its thickness, the'majority of one face of'thecrys taldevelops a positive charge while the majority of the opposite facedevelops'a' negativecharge. The majority ofthe'volume"ofthe"'crystahthen escapes has-"the polarity corresponding tothe charge d'evelopmentjuststated, which may be'called Polarity- A," theremainder of the crystal being of theoppositepolarity. After treatmentthe greater part of the above-mentioned majority of the. volume of thecrystal may have changed to the opposite polarity, which may be calledPo= larity Bf" givingthe opposite charge development so thatit is now ofthe same polarity as weenie minority of the volume of thecrystal beforetreatment. Then, in estimating the said proper f 'tion'inthe manneraforesaid, the-proportion be tion, orsig n, within'the volumewhichisrequired to -be a'ifected -by=the treatment. For-this reason changesof sign of-the temperature gradient in the crystal 's'hould be avoidedas far as possible if the strain is produced thermally. Similarly, if:it-is desired to produce-the strain mechanically, itIisgener-a'llyundesirable'todo so by bending the crystal, since-thisresults in a change of the sign 6f the strain as the neutral axis ofbending is crossed,- and there is aregion in the-neighborhood oithe-neutral axis where the stresses are lowand the desiredchanges-maynot be-produced. Such flexure, however, may'sometimes beused, if desired, in'cases where the-user is prepared to cut azsm'allercrystalirom-the crystal which has been treated and todiscard theremaining material. The preferred method'of imposing mechanical strainis=to=apply a torque in a plane perpendicular to the longestor'longitudinal direction .of

the crystal barorplateto be treated, and it will be observed that thisproduces "a strain which is of constant-sign throughout the crystalexcept,

perhaps; in the neighborhood 'of the points at which' the'torqueis-applied to "the plate. Where a-torquefis' applied, our experiencehas'been that lthe existence of a temperature gradient of theorder-of-1-C. to'2-C. per'cnrin the crystal'isgenerallydisadvantageous,and it isadvisable asa "practical matter to ensure that the temperaturedf the crystal isas nearly uniform from pointpto "point "of the'crystalas can conveniently be, secured, even though specimens of quartzoccasion- "allyare found"in'thetreatment-of which a small temperaturegradient'along the direction perpendicular-to the-plane inwhichthe'torque is appliedappearsto'be' advantageous.

It has already been stated'that treatment-of 'the kind here'proposed isnot invariably successful'. Success depends, inter"alia,-'on"the 'cut'adopted Tor-the crystal= plate. It isto be'understoo'd in connectionwith the claims hereinafter set out thattreatmentby aniapplie'd torqueonly. of

plat'esso cut that '0 is substantially 0 or ,,or

sothat 'o is substantially 30, is excluded from their scope, sincejlates.o'f'suchrcuts cannot be "effectively treated'by'the application of atorque. The'changepro'duced by'treatment in accordanceWiththE'iIlVEIltiOIl/JO decrease the proportion of cases of successfultreatment is to be expected unless the quartz is substantially pure andis free from inclusions or other mechanical defects, such as veil andbubbles, for example. Nevertheless, piezo-electric crystals normally aremade of a substantially pure quartz, such as Brazilian quartz, which isfree from flaws, so that this does not form any serious limitation onthe commercial usefulness of the invention.

It will be appreciated that the precise amount of strain which must beimposed depends both upon the individual sample which it is desired totreat and upon the particular out which has been adopted. The mostconvenient amount of strain therefore must be determined by trial forany given sample and cut, but after a little experience with any givenout the user usually will be able to forecast the appropriate amount ofstrain without any real difiiculty. An adequate working rule can,however, be formulated. In the case where the strain is imposedmechanically, a few test specimens having the crystallographicorientation in question are cut from the crystal and strainedmechanically until they fracture. The crystals to be treated are thenstrained, during the process of thermal treatment in accordance with theinvention, by an amount which is about 30% less than the strain requiredto produce fracture, If the reduction in twinning so produced isinsufficient, the crystals may be treated again under a somewhat largerstrain. It will also be appreciated that an unnecessarily large strainis undesirable, since there is a danger that crystals may fractureduring the thermal treatment, and reasonable adjustments of the amountof strain may be made by trial so as to minimize the risk of breakage.In the case where the strain is produced by means of a temperaturegradient in the crystal, a similar ,rule may in principle be applied:namely, that it is desirable that the gradient should be as large aspossible without being large enough to produce fracture of the material.

The method of treatment in accordance with the invention may be carriedout on slices of crystal which are not in the final form. Generally, itis convenient to use a slice having faces rectangular in shape, and ifthe crystal element in its final form is to have rectangular faces, itis convenient to reduce the element to its final shape before treatment,but somewhat oversize, since untwinning is often somewhat imperfectalong the edges and the oversize element can then be trimmed to correctsize and the imperfect material removed during trimming. If the finalcrystal is to be circular in shape, it nevertheless volume of thecrystal which is electrically twinned may be considerably reduced, andhence the improvement in accordance with this aspect of the inventionobtained. In some cases. however, the electrically twinned proportionmay actually be increased. This is the case when the crystal isoriginally free from electrical twinning,

for the aforesaid characteristic patterns of elecc trical twinning arestill "obtained with such crystal." Of course, if the'crystal isoriginally substantially free from electrical twinning, there is no needto apply to it the treatment in accordance with the invention unless itis desired deliberately to introduce the characteristic pattern ofelectrical twinning into the crystal. The latter may sometimes be thecase, for some of the patterns correspond to possible modes ofpiezoelectric vibration of the crystals and the presence of suchelectrical twinning enables the crystals to be excited into vibration inthese modes without the necessity of any special electrode arrangements.

In any case, whether the electrically twinned proportion is increased ordecreased, the advantage may be obtained that by localization of theelectrical twinning in the characteristic pattern it becomes possiblethereafter to cut from the treated crystal in accordance with thispattern at least one smaller section of practicallyuseful size which issubstantially free from electrical twinning. Thus it may even bepossible in some cases to make use without any waste at all of quartzwhich might otherwise have been completely useless.

Figs. Sci-9g of the drawings illustrate typicalcharacteristic electricaltwinning patterns which are obtained after the temperature strain methodof treatment aforesaid, the electrically twinned parts remaining aftertreatment being indicated by shading and the nature of the differentcuts indicated at the side of each figure. The figures show crosssections at right angles to the length of the crystal plates, thetwinning being substantially the same for parallel cross sectionsthroughout the length.

It will be appreciated that untwinned crystals of considerable anduseful sizes may be cut from the treated crystals showing these patternsof' twinning. Thus, for example, untwinnecl plates may be cut from theplates shown in Figs. 9a-9e, as indicated by the dotted lines, the plateof Fig. 96, for example, yielding two smaller plates exhibiting oppositepolarities during use. Again, the plates shown in Figs. 9 f and 9g maybe completely cut up into four and six untwinned plates respectively.

It will also be appreciated that the plate shown in Fig. 9f or By has apattern of electrical twinning modified to obtain in the crystal apattern of electrical twinning corresponding to a ossible mode ofpiezoelectric vibration of the plate and may be used without furthercutting in association with normal electrodes'for exciting such a modeof vibration.

While the'characteristic pattern is often obtained with one treatment ofa plate in this way, it may sometimes be necessary, after completing onecycle of treatment, comprising heating followed by ccoling while thecrystal is strained by maintaining a temperature gradient therein, torepeat this cycle of treatment one or more times for' the number ofcycles necessary to provide in the crystal a substantially stablecharacteristic pattern. The term stable implies that further treatmentdoes not appreciably alter the pattern of the electrical twinning.

As an example of carrying out the temperature gradient method oftreatment in accordance with the invention, let it be required to treatarectangular crystal element 50x12x5 mm. of X rectangular bar or rod ofiron 11x2.5x1;6 cm.

having a hole l1xl.2x0.3 cm. running centrally" along its entire length.'A copper tube containing a thermocouple is attached to one end of therod, and a length of 5 cm. at the other end of the rod is covered by aheating coil wound on thin mica wrapped around the rod. The coil is madeof about 200 cm. of No. 26 S. W. G. resistance wire of nickel-chromiumalloy, having a room-temperature resistance of about 12 ohms. Asbestoscord is wound around the outside of the wire. The crystal slice isplaced centrally in the cavity in the rod and the whole is placed in amufile furnace. A current of about 1.1 amperes is passed through thesaid heating coil, so as to raise the mean temperature of the rod about100 C. above that of its surroundings and to establish a temperaturegradient along its length of about 1.5 C./cm. The mufiie furnace is thenraised to such a temperature that the temperature registered by thethermocouple is about 660 C., the time taken to attain this temperature,starting from room temperature, being from three to four hours. Thetemperature rise during the first hour may be conveniently about 400 C.

The temperature of 660 C. is maintained for about A; hour, and thetemperature of the muirie furnace is then steadily reduced to 400 C.during a period of about 3 hours. During this time the crystal iscooling through the neighborhood of the inversion temperature. Asomewhat faster rate of cooling through this region may be used, but,particularly with parallelepipedal crystals not more than fivemillimeters thick, it is preferable that such cooling be at a rate ofless than 150 0. per hour. After about 400 C. is reached the temperaturemay be allowed to drop more rapidly, so that the muffle furnace returnsapproximately to room temperature in a further time of about 1 hours,the current passing through the said heating coil being switched ofiafter about an hour of this cooling time has elapsed. Thus during mostof the cooling period, the crystal is enclosed in the cavity so as to bein a metal container along which a temperature gradient is maintained toproduce a corresponding appropriate temperature gradient in the crystal,causing the crystal to be strained.

As an example of carrying out the mechanical torque method of treatmentin accordance with the invention, let it be required to treat arectangular crystal element 5x1x0.1 cm. of BT cut, corresponding to =0,:140", and =0. A suitable apparatus for applying a mechanical couple tothe plate to strain it during the heat treatment is shown in Fig. 10. I

The apparatus consists of two pieces of U-section channel iron 43 and44, fastened together on bars 45, 46 and parallel to one another at adistance apart somewhat greater than the length of the crystal plate 4'!which is to be treated. A vertical slot 48, about a millimeter widerthan the thickness of the plate 41, is cut in the inner side of channel44, and in channel 43 opposite slot 48 two 120 Vs are cut at 49 and 50.

An iron head 5| is provided with a knife edge which rides in the Vs 49and 50 after the fashion of a balance. The angle made by the sides ofthe knife edge is about 90. A groove 52 is cut in the side of head 5|which faces the slot 48. The groove 52 is about 3 millimeters deep and amillimeter wider than the thickness of the plate 41. A hole 53 about 1cm. in diameter is drilled in head 5|, as shown.

The crystal plate 41, the'ends packed in mica, not shown, is held in theslot 48 and-the groove 52. The mica serves not only to distribute the-12 pressureon the crystal 41, but also to avoid unduly large temperaturegradients where it presses against the iron. It is important to see thatthe crystal plate is accurately aligned in order to minimize the risk ofits breaking.

A constant torque may conveniently be applied to the crystal byinserting an iron rod, not shown, some 13 cm. long in the hole 53, thegreater part of the rod being on one side of the head 5!, so that mostof the weight is applied to twist the crystal. When a greater torque isrequired, iron rings may be hung on the rod at any distancefrom theknife edges. For plates 5 x 1 x 0.1 cm. a torque of 170 gm. wt. cm. hasnormally been used, though for thicker plates this has been increasedroughly in proportion to the square of the thickness of the plates.

This apparatus, holding the crystal and applying a constant torque toit, is put in a mufile furnace and the temperature raised to about 700C., the time taken to reach this temperature being, as before, aboutfour hours with an initial rise of some 400 C. per hour. The furnace isthen allowed to cool down, the initial rate of cooling being about orrather less than 100 C. per hour.

It appears desirable, for the sake of completeness, to set out rathermore fully the nature of the changes which the mechanical torque processin accordance with the invention is capable of producing. These changesare illustrated diagrammatically in Figs. 11a, 11b, 11c and 12a, 12b,and 120 for a rectangular plate cut parallel to an R face of the maincrystal, and in Figs. 13a, 13b, 13c and 14a, 14b, and 140 for a platecut parallel to an r face of the main crystal. In each case the longedges of the plate are parallel to an electric axis. Each of Figs. 11a,11b, and 110 is a plan view of the plate, a twinned area beingrepresented conveniently by a triangle. In Fig. 11a the triangular areais supposed to be electrically twinned with respect to the areaenclosing it, while in Fig. 1122 the triangular area is assumed to beoptically twinned. In Fig. 110 the triangular area is supposed to beboth electrically and optically twinned (compound optical twinning). Thedirection of the X-axis is indicated, as are also the signs of thecharges producedin the several areas by compression along the X- axis.If the several plates are strained by a couple applied about the centerline of each plate lying in the direction of the X-axis, and the thermaltreatment above described is applied, the struc-. tures after thetreatment are those shown in Figs. 12a, 12b, and 120, respectively.

It will be seen in Fig. 12a that the main body 54 of the plate remainsunaffected, remaining an R plane in the crystal; but the electricallytwinned portion 55 has been converted from an 1' plane to an R planehaving the same orientation as the remainder of the plate. The plate hasthus become homogeneous, since both portions were assumed to beoriginally of the same optical activity and, as previously explained,the treatment does not alter the sense of the optical activity. Itlikewise willbe seen from a comparison of Figs. 11b and 12b that, theoptical activity being unaffected by the treatment and there having beenno electrical twinning originally, there is no change as a result of thetreatment. The process is, in fact, as earlier remarked, not applicablefor the production of changes in crystals which are optically twinnedbut not electrically twinned. Finally, on comparing Figs. 11c and 120,it will be seen that the electrical twinning has been removed but the 13optical twinning remains. The appropriate change in accordance with theinvention has thus been produced, although, with the twinning defectillustrated in Fig. 110, the change is likely to be of little or nointerest from the commercial point of view. This, however, is of littleimportance in view of the rarity of compound optical twinning.

On the other hand, comparing Figs. 13a and 14a, showing plates beforeand after treatment respectively, it will be seen that before thetreatment a plate cut parallel to an 1- face of the main crystal andsuffering from electrical twinning only has the main body 56 of theplate changed from being an r plane to being an R plane, while thetwinned portion 51, which is in this'case an i R plane initially,remains unchanged. In Figs.

13b and 14b it will be seen that both portions of the plate wereinitially r faces and bothlarecon- Finally, from Figs. 13c and 140, itwill be seen that the appropriate change has again been produced,electrical twinning having been removed but the optical twinningremaining unchanged.

A change of this kind lacks interest from the commercial point of viewjust as does that illustrated with reference to Figs. 11c and 12c.

A further remark may be made regarding crystals treatable in accordancewith the invention for which =0 and which are originally so cut that forone part of the crystal 0=0', where 0' is greater than 0 and less than90. In an electrically twinned part of such a crystal 0 is equal to180-0'. After the treatment, the part, or a portion of the part, forwhich 0:0 will have been converted to 0=l800. This information, withthat given in connection with Figs. 11, 12, 13, and 14, will besufficient to enable those skilled in the art to estimate the probableeffect of the thermal treatment on most of the cuts generally employedin practice, and therefore will assist in making a decision as to how tocut any actual crystals which manifest twinning but which appear likelyto be worth treating.

In Figs. 15a, 15b, and 150 and 16a, 16b, and 160, cases are shown inwhich a rectangular plate is cut parallel to an 1' face of the crystal.the long edges of the plate being inclined at 45 to the electric axiscontained in the respective r plane. Figs. 15a, 15b, and 150 illustratethe material before treatment, and Figs. 16a, 16b, and 16c,respectively, illustrate the material after treatment under a torqueapplied aboutan axis parallel to the long edges of the plate. In view ofthe explanations already iven the nature of the changes will be evidentfrom the drawing. On comparison of Figs. 15a and 16a with Figs. 13:; and14a, respectively, it will be seen that whereas the 7' face in Fig. 13ais converted by the treatment to an R face, the twin R region beingunaffected, in Fig. 15a it is the twin R region which is converted to an1' face, the original 1' face remaining unafiected. This is due to thedifference in the angles between the torque axis andthe' 1 electric axisin the two cases.

It will be appreciated from this that the relation between the directionof the axis about which torque is applied and the directions of thecrystal axes has an important effect on the nature of the changesproduced. For practical purposes, it should be noted that, if it isdesired that the plate after treatment shall be of BT cut, the plateshould initially be out either as a BT plate or as a plate for which 0is the supplement of that ,finally required, it being chosen in theneighborhood of 0 whichever initial cut is chosen. In cases where it isdesired that the plate after treatment shall be of AT cut, the plateshould initially be out either as an AT plate, or as a plate for which 0is the supplement of that finally required, except that ll in this caseshould be chosen in the neighborhood of 45, whichever initial cut ischosen. After the thermal treatment of the invention the treated platemay be subdivided into piezoelectric elements having faces exhibitingthe desired value of the angle 11!.

While there have been described what are at present considered to be thepreferred embodiments of this invention, it will be obvious to thoseskilled in the art that various changes and modifications may be madetherein without departing and scope of the invention.

What is claimed is:

1. The method of treating a quartz crystal which manifests electricaltwinning comprising,

heating the crystal, and thereafter cooling said crystal while saidcrystal is strained to modify the pattern of said electrical twinning.

2. The method of treating a quartz crystal which manifests electricaltwinning comprising, heating the crystal, and thereafter cooling saidcrystal while said crystal is strained to reduce the proportion of thevolume of said crystal which is electricall twinned.

3. The method of treating a quartz crystal which manifests electricaltwinning comprising, heating the crystal to a temperature appreciablyabove its alpha-beta inversion temperature to convert it to the betaform, and thereafter cooling said crystal through said inversiontemperature while said crystal is strained to modify the pattern of saidelectrical twinning.

4. The method of treating a quartz crystal which manifests electricaltwinning comprising, heating the crystal to a temperature appreciablyabove its alpha-beta inversion temperature to convert it to the betaform, and thereafter cooling said crystal through said inversiontemperature while said crystal is strained to reduce the proportion ofthe volume of said crystal which is electrically twinned.

5. The method of treating a quartz crystal which maifests electricaltwinning comprising, heating the crystal, and thereafter cooling saidcrystal, while said crystal is strained by maintaining a temperaturegradient therein, to modify the pattern of said electrical twinning.

6. The method of treating a quartz crystal which maifests electricaltwinning comprising, heating the crystal to a temperature appreciablyabove its alpha-beta inversion temperature to convert it to the betaform, and thereafter cooling said crystal through said inversiontemperature, while said crystal is strained by maintraining: atenjlperaturegradient therein, .to modiav fy the pattern of said;electrical twinning.-

h The; method: f: ea in qu rtz ry l whichxmanifests: electrical;twinning. comprising,

heating; the" crystal; and; thereafter cooling said crystal, whilesaidgcrystal is; enclosed iii-a1- metal container alongwhichatemperature; gradient ismaintained to produce a correspondingtemperature gradient insaid crystal causingsaid? crystal tobe strained;to. modify; the; pattern ofsaid electrical twinning-.-

82 The method of; treating a quartz.- crystal whichi manifestselectrical twinning comprising, heating t'ne crystal toatemperaturegappreciabiy above; its. alpha-beta inversion temperature toconvert .it to the beta-formand thereafter cooling; saidcrystal vthrough the neighborhood of saidinversion temperatureat arate '0f:1essthan- 150' degrees centigrade perhour; while said crystahisstrained, tomodify thepajttern of said 20 electrical twinning;

9-, The method of treating a quartz; crystal which manifests electricaltwinning comprising, heating the crystal,- and thereafter coolingsaidcrystal, while said crystal is strained. by direct application! theretoof a mechanical stress, to modifythe patterniof said-electricaltwinning- 10;- The, method: of: treating a quartzv crystal which:manifests, electrical twinning comprising;

heating the-crystal, and thereafter cooling said ing the crystal, andthereafter cooling said crys-.

tal, while said crystal is strained by direct-ape plication thereto of amechanical torque, applied. in a plane. perpendicular to saidlongitudinal. direction of said crystal, to modify the pattern .of:

said electrical twinning.

12. The method of treating a quartz crystal which manifests electricaltwinning comprising,

heating the crystal, and thereafter cooling said.

crystal, while said crystal is strained by main-- taining a temperaturegradient therein, to com-.

plete one cycle of treatment modifying the pattern of said electricaltwinning, and repeating saidv cycle of heating followed by coolingwhile. said crystal is strained for the number of cycles necessary toprovide in said. crystal a substana tially stable characteristic patternof electrical twinning.

13. The method of. treating a quartz crystal which manifests electricaltwinning comprising,- heating the crystal, thereafter cooling saidcrystal while said crystal is strained to modify the pattern of saidelectrical twinning, and thereafter cutting from said crystal'at leastone'section substantially free from electrical twinning.

NORA ANNAWOOSTER; WILLIAM ALFRED WOOSTER. EDWARD ARMITAGE FIEIJDING.

REFERENCES CITED The following references are of record in the file ofthis patent:

The Properties of Silica, by Sosman, chapter- XII, pages 192-496.

Secondary Dauphine Twinning in Quartz, by Clifford Trondel fromlTheAmerican Mineral- 0gist, vo1. 30, May-June 1945, pages 447460 inclusive.

